Materials chapter 9

limitations of plain-carbon steels

- cannot be strengthened beyond about 100,000 psi (690 MPa) without a substantial loss in ductility and impact resistance - large-section thicknesses of these cannot be produced with a martensitic structure throughout, that is, they are not deep-hardenable - have low corrosion and oxidation resistance - medium-carbon ones must be quenched rapidly to obtain a fully martensitic structure. rapid quenching leads to possible distortion and cracking of the heat-treated part - have poor impact resistance at low temps

full annealing

- hypoeutectoid and eutectoid steels are heated in the austenite region about 40°C above the austenite-ferrite boundary, held the necessary time at the elevated temp, and then slowly cooled to room temp, usually in the furnace in which they were heated - microstructure consists of proeutectoid ferrite and pearlite - for hypereutectoid steels, it is customary to austenitize in the two-phase austenite plus cementite (Fe3C) region, about 40°C above the eutectoid temp

martempering process

consists of: 1. austenitizing the steel 2. quenching it in hot oil or molten salt at a temp just slightly above (or slightly below) the Ms temp 3. holding the steel in the quenching medium until the temp is uniform throughout and stopping this isothermal treatment before the austenite-to-bainite transformation begins 4. cooling at a moderate rate to room temp to prevent large temp differences the steel is subsequently tempered by the conventional treatment - microstructure is martensite / tempered martensite

low-alloy steels

containing from about 1% to 4% of alloying elements - commonly referred to simply as alloy steels - mainly automotive and constructive type steels

Fe-Fe3C phase diagram

contains the following solid phases: - α ferrite - austenite (γ) - cementite (Fe3C) - δ ferrite

critical cooling rate

cooling at a rate faster than curve E will produce a fully hardened martensitic structure

ductility and toughness of Fe-C martensites

decrease as the carbon content is increased

microstructure of Fe-C martensites

depends on the carbon content of the steel - if the steel contains less than about 0.6% C, the martensite consists of domains of laths of different but limited orientations through a whole domain - the structure within the laths is highly distorted, consisting of regions with high densities of dislocation tangles - as the carbon content of the Fe-C martensites is increased to above about 0.6% C, a different type of martensite, called plate martensite, begins to form - above about 1% C, Fe-C alloys consist entirely of plate martensite

eutectoid reaction

- at this point, solid austenite of 0.77% C produces α ferrite with 0.022% C and Fe3C (cementite) that contains 6.67% C - this reaction, which occurs at 727°C, can be written as γ austenite (0.77% C) → α ferrite (0.022% C) + Fe3C (6.67% C)

normalizing

a heat treatment in which the steel is heated in the austenitic region and then cooled in still air - microstructure of thin sections of hypoeutectoid plain-carbon steels after this consist of proeutectoid ferrite and fine pearlite

martensite in plain-carbon steels

a metastable phase consisting of a supersaturated interstitial solid solution of carbon in body-centered cubic iron or body-centered tetragonal iron (the tetragonality is caused by a slight distortion of the BCC iron unit cell)

spheroidite

a mixture of particles of cementite (Fe3C) in a α ferrite matrix

pearlite

a mixture of α ferrite and cementite (Fe3C) phases in parallel plates (lamellar structure) produced by the eutectoid decomposition of austenite - since the solubility of carbon in α ferrite and Fe3C changes very little from 727°C to room temp, this structure will remain essentially unchanged in this temperature interval

martempering (marquenching)

a modified quenching procedure used for steels to minimize distortion and cracking that may develop during uneven cooling of the heat-treated material

eutectoid steel

a plain-carbon steel that contains 0.77% C - since an all-eutectoid structure of α ferrite and Fe3C is formed when austenite of this composition is slowly cooled below the eutectoid temperature

hypoeutectoid steel

a plain-carbon steel that contains less than 0.77% C

hypereutectoid steel

a plain-carbon steel that contains more than 0.77% C

martensite

a supersaturated interstitial solid solution of carbon in body-centered tetragonal iron - a metastable structure and decomposes upon reheating

alloy steels

have developed to overcome the deficiencies of plain-carbon steels - contain alloying elements to improve their properties - in general cost more than plain-carbon steels, but for many applications they are the only materials that can be used to meet engineering requirements - the principal alloying elements added are manganese, nickel, chromium, molybdenum, and tungsten - other elements that are sometimes added include vanadium, cobalt, boron, copper, aluminum, lead, titanium, and columbium (niobium)

isothermal transformation (IT) diagram

by repeating the same procedure for the isothermal transformation of eutectoid steels at progressively lower temps, this can be constructed. and from experimental data - the S-shaped curve next to the temperature axis indicates the time necessary for the isothermal transformation of austenite to begin - the second S curve indicates the time required for the transformation to be completed

austenitizing

if a sample of a 0.77% (eutectoid) plain-carbon steel is heated to about 750°C and held for a sufficient time, its structure will become homogeneous austenite - if this eutectoid steel is then cooled very slowly to just above the eutectoid temperature, it will remain austenitic - further cooling to the eutectoid temperature or just below it will cause the entire structure to transform from austenite to a lamellar structure of alternate plates of α ferrite and cementite (Fe3C)

formation of Fe-C martensite

if a sample of a plain-carbon steel in the austenitic condition is rapidly cooled to room temp by quenching it in water, its structure will be changed from austenite to martensite

lath martensite

if the steel contains less than 0.6% C

plate martensite

if the steel contains more than 0.6% C

δ ferrite

the interstitial solid solution of carbon in δ iron - has a BCC crystal structure like α ferrite but with a greater lattice constant - the maximum solid solubility of carbon in this is 0.09% at 1465°C

hardness

(of a steel) its resistance to plastic deformation, usually by indentation

hardenability

(of a steel) that property which determines the depth and distribution of hardness induced by quenching from the austenitic condition depends on: - the composition of the steel - the austenitic grain size - the structure of the steel before quenching

eutectic reaction

- at this point, liquid of 4.3% forms γ austenite of 2.11% C and the intermetallic compound Fe3C (cementite) which contains 6.67% C - this reaction, which occurs at 1148°C, can be written as: Liquid (4.3% C) → γ austenite (2.11% C) + Fe3C (6.67% C) - this reaction is not encountered in plain-carbon steels because their carbon contents are too low

hypoeutectoid plain-carbon steels

- if a sample of a 0.4% C plain-carbon steel (hypoeutectoid steel) is heated to about 900°C for a sufficient time, its structure will become homogeneous austenite - then, if this steel is slowly cooled to about 775°C, proeutectoid4 ferrite will nucleate and grow mostly at the austenitic grain boundaries - if this alloy is slowly cooled from temp b to c, the amount of proeutectoid ferrite formed will continue to increase until about 50% of the austenite is transformed - when the steel is cooling from b to c, the carbon content of the remaining austenite will be increased from 0.4 to 0.77% - at 727°C, if very slow cooling conditions prevail, the remaining austenite will transform isothermally into pearlite by the eutectoid reaction austenite → ferrite+ cementite - the α ferrite in the pearlite is called eutectoid ferrite to distinguish it from the proeutectoid ferrite that forms first above 727°C

strengthening of Fe-C martensites

- low-carbon Fe-C martensites are strengthened by a high concentration of dislocations being formed (lath martensite) and by interstitial solid-solution strengthening by carbon atoms - the high concentration of dislocations in networks (lath martensite) makes it difficult for other dislocations to move - as the carbon content increases above 0.2%, interstitial solid-solution strengthening becomes more important, and the BCC iron lattice becomes distorted into tetragonality - however in high-carbon Fe-C martensites, the numerous twinned interfaces in plate martensite also contribute to the hardness

steels

- plain-carbon steels are essentially alloys of iron and carbon with up to 1.2% carbon - however, the majority of steels contain less than 0.5% carbon - most steels are made by oxidizing the carbon and other impurities in the pig iron until the carbon content of the iron is reduced to the required level

for martensitic plain-carbon steels w more than 0.2% C

- the main mode of carbon redistribution at tempering temps below 200°C is by precipitation clustering - in this temp range, a very small-sized precipitate called epsilon carbide forms - the carbide that forms martensitic steels are tempered from 200°C to 700°C is cementite, Fe3C - when the steels are tempered between 200°C and 300°C, the shape of the precipitate is rodlike - at higher tempering temps from 400°C to 700°C, the rodlike carbides coalesce to form sphere-like particles - tempered martensite that shows the coalesced cementite in the optical microscope is called spheroidite

in lath martensites of low-carbon plain-carbon steels

- there is a high dislocation density - these dislocations provide lower-energy sites for carbon atoms than their regular interstitial positions - thus when low-carbon martensitic steels are first tempered in the 20°C to 200°C range, the carbon atoms segregate themselves to these lower-energy sites

purposes for normalizing

- to refine grain structure - to increase the strength of the steel (compared to annealed steel) - to reduce compositional segregation in castings or forgings and thus provide a more uniform structure

plates in high-carbon Fe-C martensites

- vary in size - have a fine structure of parallel twins - often surrounded by large amounts of untransformed (retained) austenite - Fe-C martensites with carbon contents between 0.6% and 1.0% C have microstructures consisting of both lath- and plate-type martensites

precipitation strengthening process

1. Solution heat treatment is the first step in the precipitation-strengthening process. Sometimes this treatment is referred to as solutionizing. The alloy sample, which may be in the wrought or cast form, is heated to a temperature between the solvus and solidus temperatures and soaked there until a uniform solid-solution structure is produced. Temperature T1 at point c of Figure 9.40 is selected for our alloy x because it lies midway between the solvus and solidus phase boundaries of solid solution α. 2. Quenching is the second step in the precipitation-strengthening process. The sample is rapidly cooled to a lower temperature, usually room temperature, and the cooling medium is usually water at room temperature. The structure of the alloy sample after water quenching consists of a supersaturated solid solution. The structure of our alloy x1 after quenching to temperature T3 at point d of Figure 9.40 thus consists of a supersaturated solid solution of the α phase. 3. Aging is the third basic step in the precipitation-strengthening process. Aging the solution heat-treated and quenched alloy sample is necessary so that a finely dispersed precipitate forms. The formation of a finely dispersed precipitate in the alloy is the objective of the precipitation-strengthening process. The fine precipitate in the alloy impedes dislocation movement during deformation by forcing the dislocations to either cut through the precipitated particles or go around them. By restricting dislocation movement during deformation, the alloy is strengthened.

ferrous alloys

alloys based on iron

nonferrous alloys

alloys based on the other metals

α ferrite

an interstitial solid solution of carbon in the BCC iron crystal lattice - carbon is only slightly soluble in this, reaching a maximum solid solubility of 0.022% at 727°C - the solubility of carbon in this decreases to 0.005% at 0°C

plain-carbon steel

an iron-carbon alloy with 0.02% to 2% C - all commercial plain-carbon steels contain about 0.3% to 0.9% manganese along with sulfur, phosphorus, and silicon impurities

austempering

an isothermal heat treatment that produces a bainite structure in some plain-carbon steels - provides an alternative procedure to quenching and tempering for increasing the toughness and ductility of some steels 1. steel is austenitized 2. then quenched in a molten salt bath at a temp just above the Ms temp of the steel, held isothermally to allow the austenite-to-bainite transformation to take place 3. cooled to room temp - final structure is bainite

Jominy hardenability test

in industry, used to measure hardenability - the specimen consists of a cylindrical bar with a 1 in. diameter and 4 in. length and with a 1/16 in. flange at one end - since prior structure has a strong effect on hardenability, the specimen is usually normalized before testing - after the sample has been austenitized, it is placed in a fixture, and a jet of water is quickly splashed at one end of the specimen - after cooling, two parallel flat surfaces are ground on the opposite sides of the test bar, and Rockwell C hardness measurements are made along these surfaces up to 2.5 in. from the quenched end

naming alloy steels

in the US are usually designated by the four-digit AISI-SAE system - the first two digits indicate the principal alloying element or groups of elements in the steel - the last two digits indicate the hundredths of percent of carbon in the steel

hardness and strength of Fe-C martensites

increase as the carbon content is increased - so most martensitic plain-carbon steels are tempered by reheating at a temp below the transformation temp of 727°C

martempering and tempering vs. conventional quenching and tempering

martempered and tempered steel has higher-impact energy values

aging

natural aging: aging the alloy at room temp artificial aging: aging the alloy at elevated temps - most alloys require artificial aging, and the aging temp is usually between about 15% and 25% of the temp difference between room temp and the solution heat-treatment temp

cooling curve B, rapid cooling

obtained by removing an austenitized steel from a furnace and allowing the steel to cool in still air - microstructure would be fine pearlite

cooling curve A, slow cooling

obtained by shutting off the power of an electric furnace and allowing the steel to cool as the furnace cools - microstructure would be coarse pearlite

process annealing

often referred to as a stress relief - partially softens cold-worked low-carbon steels by relieving internal stresses from cold working - usually applied to hypoeutectoid steels with less than 0.3% C - carried out at a temp below the eutectoid temp, usually between 550°C and 650°C

method used to forge plain-carbon steels

oxidizing carbon and other impurities in pig iron until the carbon content of the iron is reduced to the desired level

cooling curve C

starts with the formation of pearlite, but there is insufficient time to complete the austenite-to-pearlite transformation. the remaining austenite that does not transform to pearlite at the upper temps will transform to martensite at lower temps starting at about 220°C - microstructure of this steel would consist of a mixture of pearlite and martensite

split transformation

takes place in two steps

cementite (Fe3C)

the intermetallic compound Fe3C - has negligible solubility limits and a composition of 6.67% C and 93.3% Fe - a hard and brittle compound

austenite (γ)

the interstitial solid solution of carbon in γ iron - has an FCC crystal structure and a much higher solid solubility for carbon than α ferrite - the solid solubility of carbon in this is a maximum of 2.11% at 1148°C and decreases to 0.77% at 727°C

basic-oxygen process

the most commonly used process for converting pig iron into steel - pig iron and up to about 30% steel scrap are charged into a barrel-shaped refractory-lined converter into which an oxygen lance is inserted - pure oxygen from the lance reacts with the liquid bath to form iron oxide - carbon in the steel then reacts with the iron oxide to form carbon monoxide - immediately before the oxygen reaction starts, slag-forming fluxes (chiefly lime) are added in controlled amounts - in this process, the carbon content of the steel can be drastically lowered in about 22 min along with a reduction in the concentration of impurities such as sulfur and phosphorous - the molten steel from the converter is either cast in stationary molds or continuously cast into long slabs from which long sections are periodically cut off - today approx. 96% of the steel is cast continuously, with about 4000 ingots still be cast individually - however about 1/2 of the raw steel is produced by recycling old steel, such as junk cars and old appliances - after being cast, the ingots are heated in a soaking pit and hot-rolled into slabs, billets, or blooms - the slabs are subsequently hot- and cold-rolled into steel sheet and plate - the billets are hot- and cold-rolled into bars, rods, and wire - the blooms are hot- and cold-rolled into shapes such as I beams or rails

precipitation strengthening

the object of this is to create in a heat-treated alloy a dense and fine dispersion of precipitated particles in a matrix of deformable metal. the precipitate particles act as obstacles to dislocation movement and thereby strengthen the heat-treated alloy

tempering

the process of heating a martensitic steel at a temp below the eutectoid transformation temp to make it softer and more ductile - the steel is first austenitized and then quenched at a rapid rate to produce martensite and to avoid transformation of austenite to ferrite and cementite - the steel is then subsequently reheated at a temp below the eutectoid temp to soften the martensite by transforming it to a structure of iron carbide particles in a matrix of ferrite

martensite finish, Mf

the temperature at which the austenite-to-martensite transformation finishes

martensite start, Ms

the temperature, upon cooling, at which the austenite-to-martensite transformation starts this temp for Fe-C alloys decreases as the weight percent carbon increases in these alloys

effects of alloying elements on the eutectoid temp of steels

the various alloying elements cause the eutectoid temp of the Fe-Fe3C phase diagram to be raised or lowered - manganese and nickel: lower the eutectoid temp and act as austenite-stabilizing elements enlarging the austenitic region of the Fe-Fe3C phase diagram (in some steels with sufficient amounts of nickel or manganese, the austenitic structure may be obtained at room temp) - carbide-forming elements (tungsten, molybdenum, titanium): raise the eutectoid temp of the Fe-Fe3C phase diagram to higher values and reduce the austenitic phase field. called ferrite-stabilizing elements

proeutectoid4 ferrite

α ferrite that forms by the decomposition of austenite at temperatures above the eutectoid temperature

eutectoid ferrite

α ferrite that forms during the eutectoid decomposition of austenite; the α ferrite in pearlite